Metastable mass analysis

ABSTRACT

Identification of metastable ion transitions with a double focusing mass spectrometer having an electric and a magnetic analyzer is accomplished by initially identifying an ion m* and subsequently modifying the electric analyzer in a manner for causing transition ions of mass m1 to be focused at an object of the magnetic analyzer. The daughter ion can then be identified and the precursor ion mo is determined from the relationship m* m12/mo. This measurement of m* and attending maintenance of ion energy levels while identifying m1 advantageously functions to maintain sensitivity and resolving power at normal operating values.

United States Patent FOREIGN PATENTS 5/1964 Great Britain 4/1969 Great Britain OTHER REFERENCES Mass Spectrometry and Its Applications to Organic Chemistry, J.'H. Beynon, Elsevier Publishing Company, 1960 pp. 11 to l5,class 250-4193 Primary Examiner-James W. Lawrence Assistant Examiner,-C. E. Church Attorney-Edward R. Hyde, Jr.

[72] Inventor Harold W. MajonJr.

Lis1e,111. [21] AppLNo. 725,752 [22] Filed May 1,1968 [45] Patented Oct.5, 1971 [73] Assignee The Perkin-Elmer Corporation Norwnlk,Conn.

[54] METASTABLE MASS ANALYSIS 3 Claims, 4 Drawing Figs.

52 user ..2s0/4i.9ME, 250/4196 [51] Int.CI. B0ld 59/44, HOlj 39/34 [50] FieIdotSeai-ch 250/419 [56] References Cited UNITED STATES PATENTS 2,947,868 8/1960 250/419 3,233,099 2/1966 250/419 3,416,073 12/1968 250/419 3,443,209 5/1969 Nelsonetal. 250/419 3,445,650 5/1969 Liebl 250/419 imam LEI/K Sal/RC5 sou/ace 0r yiccam n ruva POTENT/HL PATENTED um Slam 3610.921

1 55 T. Jay

i 1 l 103 h .95 104 7y I I I I INVENTOR.

Z l Harold M Ha 01", .l/r

Q/w Q/ Q f METASTABLE MASS ANALYSIS This invention relates to mass spectrometry. The invention relates more particularly to an improved mass spectrometric method and means for identifying components of a sample.

In a mass spectrometer molecules of a vaporized sample are bombarded by an electron beam and ions thereby formed are accelerated by an electric field through an analyzer section of the spectrometer toward a target. The accelerated ions traverse a time varying field in the analyzer section, the field strength of which is varied in a manner for scanning a mass spectrum by focusing ions of differing m/e values at an output aperture in the target. Focusing is accomplished in one arrangement through mass selection with a time varying magnetic field. lons passing through the output aperture excite a particle multiplier and an output signal thereof is amplified for application to a recording means such as a chart recorder. The output signal thus provided comprises a spectrum of peaks occurring successively in time, each of which is indicative of the relative abundance of a sample component.

In a sector fonn of mass spectrometer employing a magnetic scanning field, the instrument resolution is largely depending upon the variation in kinetic energy of the ions entering the scanning field. This ion energy is given by K.E.= /zmv*=eV,,, (l) where e is the charge on the ion,

V,, is the accelerating potential,

m is the mass of the ion. and

v is the velocity of the ion.

Resolution is enhanced by additionally including an electric analyzer in the ion path for establishing an electrostatic field which in conjunction with the magnetic-scanning means provides double focusing of the accelerated ions. The ions then initially traverse an electrostatic field established by the electric sector. In operation, the electric sector field causes ions of a given mass to exhibit a nearly constant energy level prior to traversing the magnetic scanning field.

It is known that ions formed by the spectrometer are subject to a secondary dissociation in the ion path existing between the accelerating electrode and the electric sector. In an exemplary ion dissociation, a precursor ion (ABC dissociates into a metastable or transition ion (AB) and an uncharged particle (C). The spectrum of mass peaks produced as a result of this dissociation are broad in appearance and usually occur at nonintegral mass numbers. Identification of metastable ion decomposition is of particular value in the structural determination of organic molecules and it is desirable that a mass spectrometer afford the means for such identification.

In a prior mass spectrometer, a double-focusing arrangement having magnetic scanning has been utilized for the identification of metastable ion transitions. The observed mass-charge number m" =m/e of ions resulting from the metastable transition with magnetic scanning is:

m l l o where m,= the mass of a precursor ion, and

m,= the mass of a daughter ion.

Although an infinite number of solutions will satisfy this relationship for the magnetic analyzer, the electric analyzer of the double-focusing spectrometer exhibits energy discrimination characteristics and facilitates identification. In a prior arrangement, the accelerating potential V,, has been altered in order to increase the kinetic energy of the transition ion. A relationship existing between particle mass and accelerating voltage V is given by:

o uo ni where al ue+ u In view of these relationships, the prior technique for identifying metastable transitions comprises magnetically scanning a mass spectrum at constant electric analyzer potential V and constant accelerating potential V,, until a desired peak believed to be a daughter peak of mass number m, is detected; maintaining the scanning field at a constant magnetic potential H adapted for focusing the peak m and measuring the accelerating potential V,,,,; increasing the accelerating potential V by the magnitude AV at constant l and magnetic field H for defocusing the first peak and focusing a second peak; and measuring the accelerating potential V at which the second peak is observed. From the data m,, V and V,,,, the mass number of the precursor ion m, can be calculated from equation (3) above.

Although this technique for identifying metastable transitions provides satisfactory results, it exhibits various disadvantages which limit the usefulness of the technique. For example, the described technique alters ion accelerating voltage which is disadvantageously accompanied by variations in ion drift time between ion source and electrostatic sector. The technique results in a loss of static resolving power and can result in reduced sensitivity under the defocused condition. In addition, there exists a practical upper limit to which the accelerating voltage can be raised thereby substantially limiting the mass range which can be searched.

Accordingly, it is an object of this invention to provide an improved method for identifying metastable ion transitions in a mass spectrometer.

Another object of the invention is to provide an improved mass spectrometer arrangement adapted for identifying metastable ion transitions.

Another object of the invention is to provide a mass spectrometer which avoids one or more of the above disadvantages.

In accordance with the general aspects of the present invention, identification of metastable ion transitions with a double focusing mass spectrometer having an electric and a magnetic analyzer is accomplished by initially identifying an ion m' and subsequently-modifying the electric analyzer in a manner for causing transition ions of mass m, to be focused at an object of the magnetic analyzer. The daughter ion can be identified and the precursor ion m, is determined from the relationship m*=m /m This measurement of m and attending maintenance of ion energy levels while identifying m, advantageously functions to maintain sensitivity and resolving power at normal operating values.

A method for identification of metastable ion transitions in a double-focusing mass spectrometer comprises focusing a peak of mass m by varying a magnetic field at constant electric analyzer potential v and at constant accelerating potential V measuring the electric analyzer potential V,,; decreasing the electric analyzer potential at constant magnetic filed H and accelerating voltage V until a different metastable peak is observed; and measuring the analyzer potential V at which the different peak is observed. The metastable transition can then be identified from the relationship:

In accordance with another feature of the invention, a double focusing mass spectrometer includes means for altering the electric sector potential over a relatively large range while maintaining the accelerating potential and magnetic field at a constant value.

These and other objects and features of the invention will become obvious from the following specifications and drawings wherein:

FIG. I is a diagram, partly in block form of a mass spectrometer constructed in accordance with features of the invention;

FIG. 2 is a block diagram of an electric potential supply means for altering the electric analyzer potential of the mass spectrometer of FIG. 1;

FIG. 3 is a detailed schematic diagram of the electric potential supply of FIG. 2; and

FIG. 4 is a view of a control panel for use with the potential source of FIG. 3.

Referring now to FIG. 1, a double-focusing sector-type mass spectrometer of Nier-Johnson geometry is illustrated. A Nier- Johnson mass spectrometer of this general type is exemplified by the Perkin-Elmer Model MS-270. The spectrometer of FIG. 1 is shown to include an ion source 10 and means I2 for sample injection means 12 is adapted to vaporize a liquid or solid sample and to provide for leaking of the vaporized sample into the ion source at a desired leak rate. Various arrangements are known for sample introduction. Although shown in FIG. 1 as coupled to the ion source by a conduit 14, the sample injector may be mounted to, and closely physically associated with, the ion source. The ion source 10 conventionally includes a source of collimated electrons for bombarding the sample molecules to thereby create positively and negatively charged ionic particles.

The ions formed by ion source 10 are accelerated toward and focused at a target 16. A source of accelerating potential, V 18 is provided and is coupled between an accelerating electrode 20, having an ion object aperture 21 located therein, and the ion source 10. The accelerated ions traverse an enclosed evacuated path defined by tubulation 22 and pass successively through electric and magnetic analyzing fields which are adapted for causing ions of a particular mass to be focused at an output aperture 23 of the target 16. Generally the electric analyzer comprises a pair of cylindrically shaped plates 24, between which a potential v, is applied. This potential is derived from a source 26 and is applied to the plates via feedthrough insulator elements 27. The electric analyzer forms an ion lens functioning to focus the ion object aperture 21 at an image point 29 which is located at an intermediate position in the ion path. Point 29 also represents an ion object for the magnetic analyzer. ions of the same mass are thereby effectively brought to a same energy level at the focus point 29 prior to entry of the ions into a magnetic scanning field, H. A magnetic analyzer comprises an electromagnet 28 adapted for establishing the magnetic scanning field in the path of the accelerated ions. The magnet 28 is excited by a scanning current of generally ramp-shaped waveform which is derived from a current source 30. An alternative current source for the electromagnet is provided by a source of adjustable DC current 32 and a switch 33 which is adapted for alternatively applying current to the electromagnet 28 from the source 32 or from the scanning current source 30. A magnetic field H having an amplitude varying with time is established by the magnet 28 when it is excited by current from the source 30 and ions of differing mass numbers at ion object point 29 are caused to be successively focused at the output aperture 23. The electromagnet 28 establishes a magnetic field of constant amplitude H when current from source 32 flows therein.

Those ions passing through the aperture 23 of target 16 impinge upon a particle multiplier 34 such as a conventional electron multiplier, the output of which is coupled to a voltage amplifier 35. The output signal from the amplifier 35 com prises a plurality of successively occurring peaks. each having an amplitude representative of the relative abundance of ions of an associated mass number. This output signal is applied to a recording means such as the chart recorder 36, which may have facilities for indicating mass number along an abscissa axis of a chart and relative ion abundance along an ordinate axis of the chart. These aspects of a mass spectrometer are well known and further elaboration is believed unnecessary.

The source of electric potential 26 is adapted for providing an adjustable electric analyzer potential. The electric analyzer functions as an energy discriminator and at constant accelerating potential V,, will focus ions at point 29 which have an energy corresponding to a particular analyzer potential V At constant accelerating potential V and an established electric analyzer potential V,.,, the kinetic energy of a precursor ion :11, is given by:

K.E. ,=eV,,= m v where v,, is the velocity of the precursor ion. When the precursor ion dissociates along that portion of the ion path between the ion source and electric analyzer into m l": +m then the kinetic energy of the transition ion m, is given by:

T =/&( "hf "I v, =%m V,, T In accordance with a feature of this invention. the electric analyzer potential is varied in magnitude to a value V for providing that transition ions In, of energy T, pass through the electric analyzer and are focused at the intermediate point 29. These ions will then be properly focused for entry into the field of the magnetic analyzer at a mass m*=m,"'/m Energy focusing of the ions m, by the electric sector provides for a peak width m, nearly equal to the peak width of the ions m, in a normal spectrum. This characteristic provides a resolving power for metastable ions under defocused conditions nearly equal to the static resolving power of the mass spectrometer under focused conditions.

In operation, an instrument operator will view a reproduced spectrum produced by a scanning current from source 30 at a constant accelerating potential v,, and at a constant electric sector potential V Upon observing a peak having characteristics indicating a metastable transition (such as a relatively broad base), the operator will establish a steady field H for focusing that particular peak m* by decoupling the current source 30 and coupling the adjustable DC current source 32 to the electromagnet 28 through the switch 33. Focusing is ac complished by adjusting the DC current amplitude in electromagnet 28 while simultaneously viewing the output of meter 38. The voltmeter 38 will provide a maximum indication when the peak m* is focused. A voltage V. and mass value m* are thus determined. The source of potential 26 is adjusted at H =k and V,,=kby decreasing the voltage V, until the original peak m* is defocused and a new peak is focused. Electric analyzer potential V is then observed at meter 39. The daughter mass m, is now determinable from equation 4 above and the precursor mass m, is determinable from equation 2.

FIGS. 2 and 3 illustrate a source of variable potential for the electric analyzer. The arrangement is particularly advantageous in that the factory V,,/ V,.., of equation 4, i.e.,

vi/ nl i is provided directly and the mass m is easily calculated by multiplying m* and this factor. The source of potential 26 includes a voltage supply 50 and a mode of operation selector circuit 52 for selecting a survey mode of operation or alternatively a measuring mode of operation. The survey circuit 54 is provided for indicating to the instrument operator the general voltage range of electric analyzer potentials within which the new peak will be focused. The measuring circuit 56 then provides for the application of the voltage V,., to the electric analyzer plate as well as a relatively accurate readout of the factor Vm/V Referring now to H6. 3. the voltage supply 50 is shown to include series coupled DC batteries 58 and 60 providing a relatively positive and a relatively negative output potential for the electric analyzer. The sum of the battery potentials comprises V the electrostatic analyzer potential for which the mass spectrometer is normally calculated. The mode selector 52 comprises a three-pole triple-throw switch 61 arranged for coupling the battery supply to the electrostatic plates 24 through survey potentiometers 62 and 64 or alternatively through the measuring voltage divider circuit arrangement 56. This measuring divider circuit arrangement comprises voltage dividing means having first and second series coupled voltage divider sections. The voltage dividing means is connected in parallel with the battery source. A first voltage divider section identified generally as 66 is coupled to an elec trostatic plate 240 and the second voltage divider section identified generally as 68 is coupled to an electrostatic plate 24b. Voltage divider section 66 includes three eight-position rotary measuring range switches 70, 74 and 72. two groups of precision resistances 76 and 78. and a precision interpolating potentiometer or helipot which is coupled between adjustable contact elements of the switches 72 and 74 The rotor switch elements are ganged and a series circuit is provided between the positive terminal of battery 58 and a common connection of resistance group 78 via the switch 61, the switch 70, a one of the resistors of group 76, the switch 72, the potentiometer 80. the switch 74 and a one of the resistances of group 78. The common circuit point of group 78 is coupled in series with the negative terminal of battery 60 through a similar arrangement including a one of a group of resistances 90, a switch 86, an interpolating potentiometer 92, a switch 84, a one of the group of resistances 88, a switch 82 and the mode selector switch 61. Potentials are derived from these voltage divider sections at adjustable contacts 94 and 96 of the potentiometer 80 and 92 respectively and are coupled to the sector plates 24. The resistive impedances of resistance groups 76 and 78 are selected for providing that counterclockwise rotation of the controls of switches 70, 72 and 74 provide a less positive potential at terminal 97 of potentiometer 80. The resistive impedances of resistance groups 88 and 90 are selected similarly for providing a less negative potential at terminal 99 of potentiometer 92 upon counterclockwise rotation of the contacts of switches 82, 84 and 86. The rotor elements of switches 82, 84 and 86 are ganged to the rotor switch elements 70, 72 and 74 while the sliding contact arms 94 and 96 are ganged for simultaneous adjustment. The potentiometers 80 and 92 provide interpolation between selected measuring switch scale positions (FIG. 4). An electric potential V,, comprising the full battery potentials is applied between the plates 24 when the switch rotors are arranged in position 1, and the slider contacts 94 and 96 are positioned adjacent terminals 97 and 99 of potentiometers 80 and 92 respectively. Other selected positions of the measuring selector switch and potentiometer contacts will provide for application of a portion of the voltage V IV, to the analyzer electrodes 24. Potentiometers 80 and 92 comprise a one-tum precision potentiometers having a common calibrated dial indicating the portion of the resistance in circuit. The resistance group switches are gang driven, thus allowing provision for a calibrated dial indicative of the resistance in circuit. A measuring selector switch indicator and interpolating potentiometer indicator thereby provide coarse and fine indications respectively of the factor GI/V.)

In order that the measured value at which a new peak is detected can be approximated, the selector switch 61 altematively couples the batteries 58 and 60 to the survey circuit 54. The survey circuit comprises a voltage divider coupled in parallel with the battery sources of potential. Slide arms 98 and 100 of the survey circuit are coupled to the sector plates 24b and 24a, respectively. These sliders are ganged and calibrated dial indicates the coarse setting for the new peak. The survey dial is correlated to the eight measuring switch positions. Potentiometers 62 and 64 are logarithmic for providing decreasing increments between scale positions on the control panel (FIG. 4) in order to provide a linear scale display. Limiter switch 102 and resistances 104 and 106 are provided for limiting the range of the survey potentiometers to that of the measuring circuit, which is on the order of V /v =9. In general the probability of metastable ion detection less than 1/9 of the mass of the precursor ion is relatively small. A second switch position however provides for bypassing of these resistances and extends the range of V /V to infinity when desired.

Focusing of the instrument is generally established for a particular ratio of V to V,. When the measuring mode is selected, variations occurring in the sector potential can alter this ratio and the focus of the instrument. Since it is desirable for reasons indicated hereinbefore to maintain the accelerating voltage V,, at a constant value, an adjusting voltage control for providing electric sector voltage adjustments is provided in order to maintain the desired ratio. This control circuit illustrated in FIG. 3 comprises a battery 106 and a potentiometer 108 having a wiper arm connected to the junction of batteries 58 and 60 via a ground circuit. Focus control voltage adjustments are made when switch 61 selects a survey or a measuring mode of operation. The wiper arm of potentiometer I08 is adjusted for establishing at the battery junction a potential for maintaining the desired ratio.

FIG. 4 illustrates a control panel for use with the electric analyzer potential supply of FIG. 3 in metastable ion examination. In an illustrative sample examination, an observed m on chart recorder 36 of FIG. 1 exhibits a value of 39.7. The survey mode is then selected and adjustment of the survey potentiometer provides a peak indication on meter 38 when the survey indicator is positioned between survey scale positions 2 and 3. A measurement mode is then selected; the measurement x switch is rotated to 2; and the interpolating y potentiometer is rotated to value of 0.14, at which value a peak is observed on meter 38. The factor V,.,/ V,,=2. i4 is read directly from the x and y scales of the panel of FIG. 4. m =2.l4 39.7= 84.958 and m,,=m, /m*=( 84,958 )/39.7=B l 82.

Thus an improved method and apparatus for analyzing metastable transitions has been described. This method and apparatus are particularly advantageous in that the ion accelerating voltage V is maintained constant during analysis, while sensitivity is maintained at a relatively high value.

While I have illustrated and described a particular embodiment of my invention, it will be understood that various modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.

I claim:

1. A method of metastable ion analysis with a double-focusing mass spectrometer having a source of ions, means for establishing an ion accelerating potential V an electric analyzer, a magnetic analyzer, an ion object aperture, an ion image located intermediate the electric and magnetic analyzers along an ion path, and an output aperture comprising the steps of:

establishing a magnetic analyzer field H and an electric analyzer field V for focusing ions of mass m*=m, /m,, at the output aperture where m,, is a precursor ion and m a daughter ion in a metastable ion transition; ionizing a sample to be analyzed; subjecting sample ions to the established accelerating potential V varying the electric analyzer at constant values of H and V,, to second value V92 at which the ions m are focused at the ion image location;

measuring and recording the values of V V, and H at which m is detected;

measuring and recording the values of V V and H at which m, is detected; and

utilizing the values m", V,.,, and V, for calculating the mass 2. The method of claim 1 wherein the electric analyzer field V, is reduced in value from V, to a lesser amplitude V 3. A method of metastable ion analysis with a double-focusing mass spectrometer having a source of ions, means for establishing an ion accelerating potential V an electric analyzer, a magnetic analyzer, an ion object aperture, an ion image located intennediate the electric and magnetic analyzers along an ion path, and an output aperture comprising the steps of:

establishing a magnetic analyzer field H and an electric analyzer field V, for focusing ions of mass m"=m, /m at the output aperture where m, is a precursor ion and m, a daughter ion in a metastable ion transition; ionizing sample to be analyzed; subjecting sample ions to the established accelerating potential V varying the electric analyzer at constant values of H and V,, to a second value V at which the ions m are focused at the ion image location;

measuring and recording the values of V V, and H at which m is detected;

measuring and recording the values of which m is detected; and

utilizing the values m, V,,,, and V, for determining the mass m,.

V V and H at 

2. The method of claim 1 wherein the electric analyzer field Ve is reduced in value from Ve1 to a lesser amplitude Ve2.
 3. A method of metastable ion analysis with a double-focusing mass spectrometer having a source of ions, means for establishing an ion accelerating potential Va, an electric analyzer, a magnetic analyzer, an ion object aperture, an ion image located intermediate the electric and magnetic analyzers along an ion path, and an output aperture comprising the steps of: establishing a magnetic analyzer field H and an electric analyzer field Ve1 for focusing ions of mass m* m12/mo at the output aperture where mo is a precursor ion and m1 a daughter ion in a metastable ion transition; ionizing sample to be analyzed; subjecting sample ions to the established accelerating potential Va; varying the electric analyzer at constant values of H and Va to a second value Ve2 at which the ions m1 are focused at the ion image location; measuring and recording the values of Va, Ve1 and H at which m* is detected; measuring and recording the values of Va, Ve2 and H at which m1 is detected; and utilizing the values m*, Ve1, and Ve2 for determining the mass m1. 